A trench capacitor is disclosed in EP 0 981 164 A2.
U.S. Pat. No. 5,541,454 A discloses the production of a semiconductor device with an electrostatic capacitive element.
Nakamura S. et al.: “Aluminum word line and bit line fabrication technology for cob dram using a polysilicon-aluminum substitute”, 1999 Symposium on VLSI Technology, Digest of Technical Papers, Kyoto, Jun. 14-16, 1999, Symposium on VLSI Technology, New York, N.Y.: IEEE, US, pages 35-36, discloses the production of an aluminum word line with a low resistance.
WO 01/17014 A1 discloses a memory cell arrangement with trench capacitors and a method for the production thereof, a capacitor electrode being formed as a metallic electrode.
Integrated circuits (ICs) or chips use capacitors for the purpose of storing charge. One example of an IC which uses capacitors to store charges is a memory IC, such as e.g. a chip for a dynamic random access memory (DRAM). The charge state (“0” or “1”) in the capacitor represents a data bit in this case.
A DRAM chip contains a matrix of memory cells which are connected up in the form of rows and columns. The row connections are usually referred to as word lines and the column connections as bit lines. The reading of data from the memory cells for the writing of data to the memory cells is realized by activating suitable word lines and bit lines.
A DRAM memory cell usually contains a transistor connected to a capacitor. The transistor contains two diffusion regions separated by a channel above which a gate is arranged. Depending on the direction of the current flow, one diffusion region is referred to as the drain and the other as the source. The designations “drain” and “source” are used mutually interchangeably here with regard to the diffusion regions. The gates are connected to a word line, and one of the diffusion regions is connected to a bit line. The other diffusion region is connected to the capacitor. The application of a suitable voltage to the gate switches the transistor on and enables a current flow between the diffusion regions through the channel in order thus to form a connection between the capacitor and the bit line. The switching-off of the transistor disconnects this connection by interrupting the current flow through the channel.
The charge stored in the capacitor decreases with time on account of an inherent leakage current. Before the charge has decreased to an indefinite level (below a threshold value), the storage capacitor must be refreshed.
Ongoing endeavors to reduce the size of storage devices foster the design of DRAMs having a greater density and a smaller characteristic size, that is to say a smaller memory cell area. In order to produce memory cells which occupy a smaller surface region, smaller components, for example capacitors, are used. However, the use of smaller capacitors results in a reduced storage capacitance, which, in turn, can adversely affect the functionality and usability of the storage device. For example, sense amplifiers require a sufficient signal level for reliable read-out of the information in the memory cells. The ratio of the storage capacitance to the bit line capacitance is critical in determining the signal level. If the storage capacitance becomes too small, this ratio may be too small to generate a sufficient signal. Likewise, a smaller storage capacitance requires a higher refresh frequency.
One type of capacitor usually used in DRAMs is a trench capacitor. A trench capacitor has a three-dimensional structure formed in the silicon substrate. An increase in the volume or the capacitance of the trench capacitor can be achieved by etching more deeply into the substrate. In this case, the increase in the capacitance of the trench capacitor does not have the effect of enlarging the surface occupied by the memory cell.
A customary trench capacitor contains a trench etched into the substrate. This trench is typically filled with p+- or n+-doped polysilicon, which serves as one capacitor electrode (also referred to as storage capacitor). The second capacitor electrode is the substrate or a “buried plate”. A capacitor dielectric, containing e.g. nitride, is usually used to insulate the two capacitor electrodes.
A dielectric collar (preferably an oxide region) is produced in the upper region of the trench in order to prevent a leakage current or to insulate the upper part of the capacitor.
What has been of great interest recently is reducing the resistivity of the electrodes of such a trench capacitor, to be precise particularly if the relevant trench capacitor is used for producing a semiconductor memory. A reduced resistance, for example of the electrode situated in the trench, makes it possible to significantly relax the requirements made of the associated selection transistor. This applies particularly to DRAM generations with gate lengths of less than 90 nm.
Hitherto, the electrode situated in the trench has generally been formed by highly doped polysilicon having a good edge coverage of more than 90% and a mediocre resistivity of more than 500*10−6 Ω cm.